1. Field of the Invention
The present invention relates to electro-acoustic transducers.
2. Description of the Prior Art
An accurate mechanoacoustic process requires that pressure waves be generated by an orderly motion of a membrane in the medium. When the medium is so displaced a pressure wave is generated. An orderly transformation thus requires that the membrane possess no preferred oscillatory modes. Two primary engineering solutions to this problem have evolved. The more direct solution utilizes a conductive membrane. Both a conductor with a substantially even distribution of electron flow and a voltage controlled electrostatic conductive membrane are moved by direct imposition of electrical forces. Without considering inevitable but controllable boundary conditions due to the support means for such elements, these elements acted upon substantially only by the medium and electrical drive forces cannot exhibit resonant mode of behavior. There are no compliances to act in concert with existing masses. Extensive structures are generally required to supply the required fields. Relatively high cost and complexity result. Structures for producing the field also interfere with radiation. Partially conductive diaphragms substantially immersed in a magnetic field allow decreased size but do not eliminate the other disadvantages.
The prior art also includes a second attempted engineering solution which utilizes a mechanical system of mass and compliance which exhibits an infinite number of oscillatory modes. Such a device is termed a transmission line. A uniform transmission line terminated resistively in the characteristic impedance with a surface in contact with the medium may also generate a pressure wave. Within practial limits set by the degree of uniformity achieved in the construction of the line and terminating impedance, orderly behavior may be achieved.
A basic distinction between these major systems is that a finite time delay is involved before a transmission line achieves steady state power transfer to the medium. A wave-train progressing along the line is increasingly brought into contact with the medium. The process is complete when the termination is reached. The medium however is also a transmission line due substantially to its uniformity.
There exists in the medium a region in which the distance to various portions of the mechanical transmission line is increasingly greater for portions closer to the sending end. At a particular angle in this region the arrival time for a pressure wave originating from the portions of the transmission line is identical. At this angle, the membrane appears to be moving in unison. Practical transmission line transducers involve obtaining a high propagation velocity while also obtaining an efficient match between driving impedance and characteristic impedence.
A pulsating cylinder provides a radiated power which decreases about 6 decibels per octave with a flat response on axis. This is superior to a flat vibrating diaphragm whose radiated power falls about 12 decibels per octave of increasing frequency. Passive diaphragms which are not characterized by transmission line behavior have resonant modes of finite Q which color the reproductive process with frequency dependent characteristics. These characteristics also deteriorate the transient behavior. Any passive mechanical member can substantially meet the definition of a transmission line requiring only that the member be uniform. Each successive incremental segment orthogonal to the axis of propagation must be substantially identical. Exceptions to this rule must have a constant characteristic impedance or frequency selective behavior will occur.
The prior art for transmission line devices discloses only relatively non-uniform constructions. Cone constructions are examples of non-uniform transmission lines. The radius of curvature of these surfaces decreases toward the large end. The propagation velocity varies as well as the characteristic impedence. Difficulty in resolving any characteristic impedence and therefore obtaining reflectionless behavior is to be expected. Operation over small axial lengths of cone reduces the problem but also reduces efficiency. A gradual resistive absorption of the progressing wave also reduces the reflective behavior at the expense of efficiency. The construction is non-uniform but it is good compromise where full-range operation is desired. A further complication occurs because the driving force of a cone is directed at an angle to the plane of the conic surface. This causes a radially symmetrical tortional stress in the cone resisted by the reactive force of the hoop or circumferential compliance. If the cone is slightly irregular or the driving force directed so as not to be radially symmetrical a distortion is propagated as a direct result. This increases tolerance requirements or results in distortions.
Certain of the disadvantages of the prior art have been partially mitigated by substitution of a soft dome arrangement for the cone arrangement described herein in detail. The dome arrangement is limited to high frequency and does not have the frequency range of the cone arrangement. Within this limitation, the dome arrangement has two advantages: the dome is more compact and less expensive.
Unfortunately, the dome arrangement is not a uniform transmission line nor does it have radial dispersion.